Opening a blocked artery with balloons and stents can be lifesaving in the early hours after a heart attack, but a new study concludes that it often does no good if the heart attack occurred three or more days before.

The findings should change medical practice, researchers say, and could affect as many as 50,000 patients a year in the United States. They say doctors should stop trying to open arteries in people who had heart attacks days or weeks before and who are stable and free of chest pain.

Currently, the balloon procedure, called angioplasty, is used in many of those patients, along with stents, devices implanted to prop open an artery. When patients receive treatment late, it is often because they did not realize that they had had a heart attack and delayed going to the doctor or hospital. In some cases, too, doctors may not make the correct diagnosis right away.

The new study “should change practice, and I believe it will,” said Dr. Judith S. Hochman, director of the cardiovascular clinical research center at New York University medical school, and leader of the study, which included 2,166 patients at 217 hospitals in the United States and other countries.

Dr. Elizabeth G. Nabel, director of the National Heart, Lung, and Blood Institute, which helped pay for the research, said: “This is an important study. It’s definitive. The evidence from it should be weighed very carefully by the groups that formulate guidelines about when to conduct angioplasty in the setting of a heart attack.”

Dr. Hochman presented the results yesterday at a meeting of the American Heart Association in Chicago. The study was also published online yesterday by The New England Journal of Medicine and will appear in the Dec. 7 issue.

Her report is the latest warning from researchers who suspect that stents are being overused and may even be causing heart attacks and deaths. Manufacturers sell $6 billion in stents a year, and hospitals charge $10,000 to $15,000 per patient to implant them.

The devices, which came into widespread use in the 1990s, are made of either bare metal or metal coated with a drug meant to help keep the artery from closing again. Recent studies have found that drug-coated stents can increase the risk of blood clots, even years after they are implanted.

Use of the drug-coated stents has fallen since last spring, but they still account for more than 80 percent of stents implanted in the United States.

Stents have been increasingly used in people with clogged arteries who are not having heart attacks, but some studies suggest that those patients would be better off taking drugs to reduce cholesterol, inflammation and clotting. The reasoning is that artery disease tends to be systemic and extensive, but stents can treat it only in tiny spots.

A million Americans a year have heart attacks, and half of them die, according to the National Heart, Lung, and Blood Institute. A common symptom is severe pain in the chest, left arm, jaw or back, but about a third of patients do not have chest pain. Symptoms may also include feeling faint, sweaty, short of breath or nauseated and having a sensation that the heart is pounding.

Clots are often the cause of heart attacks, and health authorities urge people with symptoms to go to the hospital immediately so that doctors can use balloons and stents, or clot-dissolving drugs, to try to prevent heart damage. But some patients do not realize what is happening, or do not want to believe it.

About a million angioplasties a year are performed in the United States, including those in heart attack victims and those in people with blocked arteries who have not had heart attacks. The study did not address the use of angioplasty and stents in people who have not had heart attacks.

Dr. Hochman emphasized that angioplasty was still the best treatment for many heart attack patients who go to the hospital early, within 12 hours of when the attack began. The people who generally do best are those who arrive at the hospital and have angioplasty within 90 minutes of the heart attack.

Opening a clogged artery in the early stages of a heart attack — “early angioplasty” — can restore blood flow and reduce damage to the heart muscle. It can sometimes save the muscle after 12 hours, but then doctors decide case by case whether it is worthwhile. About a third of heart attack patients show up after 12 hours.

After a few days have passed, though, there is little or no chance of preventing heart damage. But many doctors open the artery anyway, thinking it may still have long-term benefits and prolong the patients’ lives. In that situation, the procedure is called late angioplasty. It can be helpful in people who are still suffering from chest pain or have certain other complications from the heart attack.